Laser diode-pumped monolithic solid state laser device and method for application of said device

ABSTRACT

The invention relates to an intracavity-doubled laser device, comprising a pumping laser-diode, a Nd:YAG amplifying medium stimulated by a laser beam with a fundamental wavelength emitted by the laser diode, the output face of said amplifying medium being cut at the Brewster angle for said fundamental wavelength and a birefringent frequency-doubling KNbO3 crystal. The device further comprises an isotropic medium ( 3 ), inserted between the input face ( 8 ) of the birefringent crystal, the amplifying medium ( 2 ) and the birefringent crystal ( 4 ), being fixed to each other such as to provide a monolithic resonant cavity. Furthermore, the crystal axis “c” of the birefringent crystal includes a non-zero angle &lt;c with relation to the orthogonal direction of polarisation of the fundamental wave defined by the Brewster surface.

The present invention relates to a laser diode-pumped monolithicsolid-state laser device, and more particularly relates to a monomodeintracavity-doubled solid-state laser. It also relates to a method usedin such a device.

The present invention can be applied particularly beneficially, but notexclusively, in the field of the generation of blue or green laserluminescence.

The laser emission of a beam of good spatial and spectral quality in thevisible spectrum at 473 nm for example, using a multimode diode, is ofgreat benefit for industrial and medical applications in particular.This wavelength, termed harmonic, can be obtained by the doubling offrequency of a laser emission at a wavelength, termed fundamental, at946 nm from yttrium aluminium garnet doped with neodymium (Nd: YAG).

Generally, an intracavity-doubled laser comprises a laser diode forpumping a solid-state laser, such as Nd:YAG for example, forming anamplifier at 946 nm. In order to produce the doubling, a non-linearcrystal is linked to the amplifier converting the near infraredfundamental signal into a visible signal by frequency doubling, (alsoknown as “second harmonic generation SHG”). A fundamental wavelengthdivided by two is thus obtained. The amplifier and the non-linearcrystal are contained in a cavity the two extreme opposite surfaces ofwhich in the path of the laser beam are reflective for certainwavelengths.

However, if a continuous emission is sought, the power of thefundamental emission is less than the power of the laser diode and thefrequency doubling is therefore very inefficient.

The American patent U.S. Pat. No. 4,809,291, entitled “Diode pumpedlaser and doubling to obtain blue light” is known, in which R. L. Byerand T. Y. Fan propose an intracavity doubling in order to increase thepower of a fundamental wave at 946 nm and thus increase the doublingefficiency.

In an article entitled “Efficient blue emission from anintracavity-doubled 946 nm Nd: YAG laser” published in 1988 in thejournal Optics Letters (vol. 13, pp. 137-139); Dixon et al. present anemission of 5 mW of blue light (473 nm) by an intracavity-doubled Nd:YAG-based microlaser. The Nd concentration is 1.1 at. %. The doublingefficiency is only 2%.

The main problem with these intracavity-doubled lasers is the presenceof axial modes and of spurious polarization which reduce the efficiencyof the laser and which are the source of high power fluctuations. As anexample, Matthews et al., in an article entitled “Diode pumping in ablue (473 nm) Nd:YAG/KNb03 microchip laser” (CLEO'96, vol. 9, p. 174)produce 26.5 mW of blue light with fluctuations of intensity greaterthan 10%.

More precisely, the intracavity frequency doubling causes selectivelosses which increase with the pumping power for the main laseremission. When the doubling efficiency increases, the average populationinversion of the cavity must increase in order to compensate for theexcess loss. However, this allows adjacent modes and the orthogonalpolarization emission to start to lase. For the adjacent modes, thiseffect is in addition to that of “spatial hole burning” which alreadyallows the adjacent modes to lase.

The different modes lasing in the cavity are coupled in the amplifyingmedium (gain competition) and in the frequency doubling medium(frequency addition). These couplings are non-linear and participate ina complex non-linear dynamic. The latter results in a high or evenchaotic fluctuation of power.

If the frequency doubling is of “Type I”, the orthogonal polarizationmodes are not subject to efficient frequency doubling (absence of phaseadaptation between the fundamental and the harmonic). These modesstabilize the population inversion by increasing with the pumping power.They slow the conversion efficiency which requires an increase of thepopulation inversion in order to increase. Only “spatial hole burning”effects allow a slight increase in the conversion efficiency.

Several methods have been presented for making the laser monomode or foruncoupling the modes in the non-linear crystal. They can be separatedinto three categories:

a) The first is the introduction of an etalon into the cavity. Thismethod, disclosed in particular in the American patent U.S. Pat. No.5,838,713 of Y. Shimoji, poses several problems. The etalon causeslosses in the cavity unless it is formed by the faces of the YAG and ofthe doubling crystal. In the latter case, it requires very greatprecision in positioning (sub-micrometric) which is difficult to obtainindustrially and to stabilize. A way of solving this problem is to bringthe amplifying medium into optical contact with the doubling crystalincorporating an angle on one portion of the contact face. This angleproduces a small air gap between the two materials. This method weakensthe contact and therefore the integrity of a monolithic laser and doesnot allow the protection of the interface by a bonding agent.

b) The second category involves the polarization of the fundamental. Theamplifying medium can be inserted between two quarter wave plates inorder to avoid the “spatial hole burning” effect, see in particular G.Hollemann et al., in “Frequency-stabilized diode-pumped Nd: YAG laser at946 nm with harmonics at 473 nm and 237 nm”, Opt. Lett. 19, p. 192,February 1994. One drawback of this method is the introduction of lossesinto the cavity.

By Type I doubling, is meant an embodiment in which the fundamentallaser beam propagates along one of the optical axes of the crystal (ingeneral the slow axis) and the harmonic laser beam propagates along theother optical axis of the crystal, orthogonal to the first. Type Idoubling occurs when it is possible to cut the crystal so that therefractive index of an optical axis at the fundamental wavelength isequal to the refractive index of the other optical axis at the harmonicwavelength. This is the case for KNbO₃.

By type II doubling, is meant an embodiment in which the fundamentallaser beam is present on the two axes and the conversion coefficient isoptimized when the polarization of the fundamental laser forms an angleof 45° with respect to the optical axes.

c) The third method consists in reducing the length of the cavity. Itwas proposed by A. Mooradian in the patent U.S. Pat. No. 5,256,164October 1993. For a linewidth of 1 nm for emission at 946 nm (comparedwith 0.6 nm for the line at 1.064 μm), Mooradian's formula requires acavity length of less than 300 μm, including the YAG and the KNbO₃. TheNd concentration in the microchips published or patented to date doesnot exceed 1.1 at. %. This corresponds to an attenuation of 0.85 mm⁻¹ at808.4 nm, i.e. 8.1% of absorbed pump power per 100 μm of thickness and15.6% of absorbed pump power per 200 μm. However, the 100 or 200 nm ofKNbO₃ do not provide adequate conversion

efficiency. Thus, a microchip laser according to Mooradian's inequalitydoes not appear to be able to emit more than a few mW of blue light withlaser diode pump power of 1 W.

Moreover, an efficient method proposed by T. Y. FAN., “Single-AxialMode, Intracavity Doubled Nd: YAG Laser”, IEEE Journal of QuantumElectronics, vol. 27, 09 Sep. 1991, is known for making anintracavity-doubled laser single-frequency. In this method, theamplifying medium (Nd: YAG) is cut at the Brewster angle with respect tothe air. The non-linear, birefringent crystal is struck at 45° by thefundamental (type II doubling). The Brewster window causes significantlosses in the orthogonal polarization and prevents it from lasing. Italso causes losses at every wavelength at which the polarization hasbeen rotated by the birefringent crystal. This loss modulation as afunction of wavelength can make the laser monomode. On the other hand,this method does not apply to a Type I frequency doubling as the signalat the fundamental frequency is on one of the optical axes of thenon-linear crystal. However, because of the double refraction, it is notpossible to join the amplifying crystal cut at the Brewster angle to thenon-linear crystal. In fact, the double refraction introduces phaseeffects which mean that the beams reflected by the external face of thecavity do not recombine when they return to the amplifier.

The present invention aims to solve most of the above drawbacks byproposing an intracavity-doubled solid-state laser which is of compactsize, provides great operating stability, and allows Type I and IIfrequency doublings. Another aim of the invention is to propose atunable solid-state laser capable of operating in monomode. Theinvention also relates to a solid-state laser which is powerful whateverthe power level of the pumping laser diode.

At least one of the above aims is achieved with a laser devicecomprising:

-   -   an optical pumping means, preferably a laser diode,    -   an amplifying medium excited by a laser beam with a fundamental        wavelength emitted by the optical pumping means, the output face        of this amplifying medium being cut according to the Brewster        angle for said fundamental wavelength, and    -   a birefringent crystal for frequency doubling.

According to the invention, the device also comprises an isotropicmedium inserted between the output face of the amplifying medium and theinput face of the birefringent crystal, the amplifying medium and thebirefringent crystal being firmly attached to each other so as toconstitute a monolithic or composite resonant cavity. Moreover, thecrystalline axis “c” of the birefringent crystal forms an angle θ_(c)which is not zero with respect to the orthogonal direction ofpolarization of the fundamental wave, defined by the Brewster surface.

With this device according to the invention, the interface between theisotropic medium and the birefringent crystal is close to the normal.This interposed isotropic medium allows the effects of the doublerefraction of the birefringent crystal to be limited: in fact, when theangle of incidence tends towards the normal, the angle of the doublerefraction tends towards zero. It is thus possible to firmly attach theamplifying medium to the birefringent crystal (the doubler) so as toobtain a compact component, which is not the case in the document of T.Y. Fan of 1991.

The combination of an interface at the Brewster angle and a birefringentcrystal which is off-axis allows a single mode to be selected. In fact,the Brewster interface causes a selective loss in the orthogonalpolarization. Only the wavelengths for which the phase shift due to thebirefringence is a multiple of 2π keep the low loss polarization at theBrewster interface. By adjusting in particular the length of thenon-linear crystal, it is possible to select only a single mode in theemission band. In other words, the index of the isotropic medium and theangle θ_(c) associated with the length of the cavity can be adjusted inorder to allow only a single mode in the cavity.

Thus, even in the case of a type II doubling, it may be expedient tochose an angle θ_(c) which is different from 45°.

Moreover, when the doubling efficiency is high, it is possible toincrease the losses of the adjacent modes by increasing θ_(c).

According to an advantageous characteristic of the invention, the inputface and/or the output face of the birefringent crystal is cut accordingto a slight angle ε with respect to the normal to the direction ofpropagation of the laser beam. Thus the input and output faces are nolonger completely parallel. This characteristic is remarkable for thefact that, in conventional doublings, the angle ε is always equal tozero in order to prevent any double refraction. Setting an angle ε whichis not zero therefore goes against conventional practices. According tothe characteristics of the device (size of the cavity, index, θ_(c)etc.) a person skilled in the art can determine a maximum angle ε beyondwhich the transmission spectrum at the Brewster face no longer has atransmission peak. By way of example, ε can be chosen to be less than orequal to 1°.

This angle ε causes a slight double refraction in the two parallel andorthogonal orientations. It can be introduced in the dimension parallelto the polarization. The slight double refraction then introduced in thepath of the signal can be compensated for by the thermal lens induced bythe pump as long as ε is slight. But preferably ε is introduced in thedimension orthogonal to the polarization.

Generally, this angle ε advantageously allows the length of thebirefringent crystal to be varied by simple translation of the pump (andtherefore of the signal). This variation in length allows tuning of thefrequency of the cavity.

According to the invention, the plane orthogonal to the direction ofpropagation of the fundamental wave can advantageously contain thecrystalline axis “c”, and form an angle with respect to the “a” and “b”axes of the birefringent crystal so as to obtain a phase matching at theoperating temperature between the fundamental wave and the harmonic wave(doubled wave).

The size of the cavity is no longer the only parameter which can bemodified in order to obtain a monomode operation. Careful choice of themedia, their refractive indices and their size and the orientation ofthe birefringent crystal, allows monomode and stable operation.

The amplifying medium can be constituted by yttrium aluminium garnet(YAG) doped with neodymium (Nd). This crystal can be cylindrical with aninput face forming a plane mirror.

The birefringent crystal is advantageously made of potassium niobate(KNbO₃).

According to a first variant of the invention, the isotropic medium is acrystalline medium made from potassium tantalate (KTaO₃). The threemedia are then joined to each other.

According to a second variant of the invention, the isotropic medium isthe air. In this case, the surface condition of the output face of theamplifier and of the input face of the birefringent crystal do notrequire excessive purity.

Preferably, care will be taken choosing an isotropic medium constitutedby an isotropic crystal the refractive index of which is close to, forexample within 10%, the refractive index of the birefringent crystal.This minimizes the double refraction effects and therefore allowsgreater tolerance regarding the striking angle of the signal withrespect to the interface (around the normal).

According to another aspect of the invention, a method is proposed inwhich the optical path length covered by the laser beam is varied bytranslating the laser beam emitted by the pumping means with respect tothe input face of the amplifier. More precisely, the laser beam is movedalong a plane in which the distance covered by this laser beam in theamplifier varies as a function of the latitude of the passage.

Other advantages and characteristics of the invention will becomeapparent by studying the detailed description of an embodiment which isin no way limitative and the attached drawings, in which:

FIG. 1 is a diagrammatic section view of a laser device comprising threecrystals joined according to the invention;

FIGS. 2 to 7 are graphs illustrating the level of power of the differentmodes prevailing in the resonant cavity of the device of FIG. 1; and

FIG. 8 is a diagrammatic section view of a variant of the deviceaccording to the invention in which the intermediate material isreplaced by the ambient air.

An intracavity frequency-doubled monolithic laser device comprising aNd: YAG laser emitting at 946 nm, with intracavity doubling at 473 nmwith a KNbO₃ crystal will now be described, although the invention isnot limited to this embodiment.

With reference to FIG. 1, a pumping laser diode 10 is seen emitting alaser beam 11 at 808 nm towards a cavity 1 composed of an assembly ofthree crystals 2, 3 and 4. The amplifying crystal 2 is Nd: YAG. Itsrefractive index is n₁=1.82 at 946 nm. The input face 6 of this crystal2 is treated so as to constitute a plane mirror. Its output face 7 iscut at the Brewster angle calculated from the index n₁ and from theindex n₂ of the isotropic crystal 3. The two crystals 2 and 3 are joinedto each other on a portion of the face 7.

The isotropic crystal 3 is constituted by potassium tantalate KTaO₃ withan index n₂ equal to 2.179 at 946 nm. On its output face 8, afrequency-doubling birefringent crystal 4 is joined, constituted bypotassium niobate KNbO₃ the refractive index and the diameter of whichare approximately identical to those of the isotropic crystal 3.

The two crystals 3 and 4 have colinear geometrical axes. The isotropiccrystal 3 is cut at the face 7 so that the laser beam 5 exiting from theNd: YAG 2 and deflected by the face 7, passes through the crystals 3 and4 parallel to their geometrical axes.

The input face 6 of the Nd: YAG 2 and the output face 9 of the KNbO₃ 4are treated in a conventional manner in order to constitute a resonantcavity. The beam exiting from the face 9 can be at 946 nm or at 473 nm.

The diagram of FIG. 1 is based on a vertical polarization of the signalat 946 nm in the Nd: YAG. The vertical axis is situated in the plane ofthe figure, the horizontal axis being perpendicular to this plane. Aperson skilled in the art will be able to easily adapt this diagram fora linear horizontal or any other type of polarization.

In the birefringent crystal KNbO₃, the input face 8 contains the axis cand cuts the plane ab according to an angle φ=32° with respect to theaxis b so as to obtain a phase matching between the wavelengths 946 nmand 473 nm at 308 K. A person skilled in the art will be able to modifythis angle for a phase adaptation at other temperatures. At 946 nm, therefractive index on the axis c is n_(3f)=2. 127 and the refractive indexon the orthogonal axis is n_(3S)=2.238. The birefringence is thereforecharacterized by Δn₃=0.111.

The angle of the face 7 φa is the Brewster angle between the YAG and theKTaO₃. It is determined by the relationship tan(φa)=n₂/n₁. The face 8 iscut at an angle ε of the normal to the direction of propagation of thebeam at 946 nm emitted orthogonally to the input face 6. φb et φc arerespectively the angles of the faces 8 and 9, and are determined byφb≅2φa−π/2+ε and φc≅2φa−π/2+ε1, ε1 having preferred values comprisedbetween ε(1n₂/n_(3S)) and ε(1−n₂/n_(3f)). The tolerance regarding φb isof the order of 1°, it is limited by the effects of double refraction.The tolerance regarding φc is less than a few angular minutes, becauseit is this latter face which closes the cavity.

Also in FIG. 1, for “y”=0, on the vertical axis, the signal propagatesover 2 mm in each of the three crystals 2, 3 and 4. The respectivepropagation distances in the three media for “y”<0 are easily deducedfrom the three angles φa, φb et φc. The optical distance Lo, product ofthe distances and the indices is easily calculated as a function of “y”.The angle ε is chosen equal to 0.003 rad or 10′ of angle. The opticalfrequencies which can propagate in the cavity are proportional to C/2Lwith C the speed of light in vacuum. They are represented by crosses andcircles in FIGS. 2 to 7.

FIGS. 2 to 7 show the losses caused by the face 7 at the Brewster anglewhen there is entry into and exit from the cavity constituted by thethree crystals 2, 3 and 4 by a signal propagating with a vertical andhorizontal polarization. The cutting angle of the birefringent crystal4, i.e. the angle of the axis c with the horizontal, is θ_(c)=0.3 rad.The amplifying medium 2 is able

to provide a gain over a band of 1 nm centered around 946.6 nm. Thelosses of all of the amplified modes are calculated then shown in FIGS.2 to 7. In FIG. 2, the orthogonal polarization losses are alsorepresented (in the form of circles). They are not represented in thefollowing, since they are too large to allow a laser oscillation.

The polarization of the fundamental signal is the vertical in the casepresented. If the axis c was horizontal, the optical axes would behorizontal and vertical. The fundamental signal would then propagatealong an axis of the birefringence and its polarization could no longerbe rotated. If the axis c and therefore the birefringence axes arerotated as in the present case, the fundamental is no longer in thebirefringence axes and its polarization is therefore rotated duringpropagation in the crystal. The mode selection by polarization rotationcan then be applied.

The vertical position “y” of the laser beam is varied in FIGS. 3 to 7.When “y” goes up from −1.4 mm to −0.8 mm, all of the wavelengths of theemission band can be selected successively. It is seen in FIGS. 4 and 5that the judicious choice of n₂, the length of the KNbO₃ birefringentcrystal 4 and of θ_(c) make it possible to select only one mode. The lowratio n₂/n₁, in particular allows a narrow width of the transmissionpeak, making the filter very selective. The angle θ_(c) can be increasedin order to bring about more losses on the adjacent modes. A verticalshift of 1.2 mm is sufficient to attain the same mode selection (seeFIGS. 3 and 7). The laser is therefore monomode and can be tuned by asimple translation of the crystals with respect to the laser diode.

In FIG. 8 a preferred variant of the device according to the inventionis represented in which the isotropic medium is constituted by the air.

The pumping device is not represented.

The output face 14 of the Nd: YAG laser 12 is cut according to theBrewster angle. An upper portion of this face 14 is intended for thepassage of the laser beam at 946 nm. On a lower portion of the face 14 abirefringent crystal 13 based on KNbO₃ is joined. The latter is cut insuch a way that the laser beam 16 exiting from the Nd: YAG and havingpassed through the air, reaches the input face 17 of this crystal 13. Inthe path of the laser beam, no material is arranged between the outputface 14 of the Nd: YAG laser and the input face 17 of the birefringentcrystal 13. The advantage of such a variant is a lesser requirement withregard to the condition of the surfaces on the bonding area 15.

Of course, the invention is not limited to the examples which have justbeen described and numerous modifications can be applied to theseexamples without exceeding the scope of the invention.

1. Laser device comprising: an optical pumping means (10), an amplifyingmedium (2) excited by a laser beam (11) with a fundamental wavelengthemitted by the optical pumping means, the output face (7) of thisamplifying medium being cut according to the Brewster angle for saidfundamental wavelength, and a birefringent crystal (4) for frequencydoubling; characterized in that it also comprises an isotropic medium(3) inserted between the output face (7) of the amplifying medium andthe input face (8) of the birefringent crystal, the amplifying medium(2) and the birefringent crystal (4) being firmly attached to each otherso as to constitute a monolithic resonant cavity; and in that thecrystalline axis “c” of the birefringent crystal forms an angle θ_(c)which is not zero with respect to the orthogonal direction of thepolarization of the fundamental wave, defined by the Brewster surface.2. Device according to claim 1, characterized in that the input face (8)of the birefringent crystal is cut according to a slight angle ε withrespect to the normal to the direction of propagation of the laser beam(5).
 3. Device according to claim 1, characterized in that the outputface (9) of the birefringent crystal is cut according to a slight angleε with respect to the normal to the direction of propagation of thelaser beam (5).
 4. Device according to claim 2, characterized in thatthe angle ε is less than or equal to one degree.
 5. Device according toclaim 1, characterized in that the plane orthogonal to the direction ofpropagation of the fundamental wave contains the crystalline axis “c”,this plane forming an angle with respect to the “a” and “b” axes of thebirefringent crystal so as to obtain a phase matching at the operatingtemperature between the fundamental wave and the harmonic wave. 6.Device according to claim 1, characterized in that the amplifying medium(2) is constituted by yttrium aluminium garnet (YAG) doped withneodymium (Nd).
 7. Device according to claim 6, characterized in thatthe amplifying medium (2) is a cylindrical Nd: YAG the input face ofwhich constitutes a plane mirror.
 8. Device according to claim 1,characterized in that the pumping means (10) is a laser diode.
 9. Deviceaccording to claim 1, characterized in that the birefringent crystal (4)is made from potassium niobate (KNbO₃).
 10. Device according to claim 1,characterized in that the isotropic medium is the air.
 11. Deviceaccording to claim 1, characterized in that the isotropic medium is madefrom potassium tantalate (KTaO₃).
 12. Device according to claim 1,characterized in that the isotropic medium is constituted by anisotropic crystal the refractive index of which is close to, for examplewithin 10% of, the refractive index of the birefringent crystal. 13.Method used in a laser device according to claim 1, characterized inthat the optical path length covered by the laser beam is varied bytranslating the laser beam emitted by the pumping means with respect tothe input face of the amplifier.
 14. Device according to claim 2,characterized in that the output face (9) of the birefringent crystal iscut according to a slight angle ε with respect to the normal to thedirection of propagation of the laser beam (5).
 15. Device according toclaim 3, characterized in that the angle ε is less than or equal to onedegree.
 16. Device according to claim 14, characterized in that theangle ε is less than or equal to one degree.